1. Field of the Invention
This invention relates to semiconductor devices and more particularly to very large scale integrated circuits operating at high speed.
2. Description of the Prior Art
Integrated circuits are well known. Within recent years, much interest has been exhibited in providing integrated circuits utilizing very large scale integration (i.e. "VLSI"). Such circuits contain a very large number of semiconductor devices, usually over 10,000, having at least 1 device per square mil. It is also quite desirable and sometimes essential to provide a VLSI circuit which is capable of operating at extremely high frequencies (e.g. above 1 gigahertz) or very high pulse rates (e.g. above 1 gigabit per second).
The following disclosure is directed to integrated circuits constructed in silicon semiconductor material. However, it will be apparent to those of ordinary skill in the art that the disclosed concepts are equally applicable to integrated circuits constructed in materials other than silicon.
Referring to the enclosed drawings, a cross-sectional view of a typical packaged prior art integrated circuit 10 is shown in FIG. 1a. The integrated circuit structure 10 comprises a semiconductor substrate 12 which is physically and electrically mounted on lead frame 11. Lead frame 11 is part of a package such as, for example, a Dual-In-Line (DIP) package. Typically lead frame 11 is fabricated of Kovar or Alloy 42, or other suitable conductive material. On the top surface of substrate 12 is formed an active region 13 containing active devices (not shown), including transistors, diodes, and logic gates. An insulating layer 14, typically comprising silicon dioxide, is formed on the surface of active region 13. Located above active region 13 and spaced apart therefrom by insulating layer 14 is an electrically conductive interconnect layer 15. Interconnect layer 15 typically is formed using a highly conductive material, such as aluminum or an alloy of aluminum, patterned to interconnect various components within active region 13 to other active components within active region 13 and to external circuitry (not shown), as desired. As is well known to those of ordinary skill in the semiconductor art, connections between conductive layer 15 and selected regions within active area 13 are provided by openings or "vias" 19 in the insulating layer 14 covering the active region 13.
During operation of the circuit shown in FIG. 1a, a signal applied between point A of the electrical interconnect layer 15 and the lead frame 11, or other common conductor, will be propagated to point B of the electrical interconnect layer 15. At high frequencies, the signal propagates from point A to B as an electromagnetic wave having traveling electric and magnetic field components in the regions between layer 15 and the lead frame 11. The electromagnetic field components and their associated voltages and currents are distributed in these regions in such a manner as to satisfy well-known Maxwell relationships relating to "skin effect". That is, at high frequencies, high frequency currents are found in the portions of the substrate 12 closest to conductor 15. The active region 13 may also play a role, but it is rarely continuous directly below 15 and may be disregarded in a first approximation.
Because substrate 12 usually has far higher resistivity than do metals such as copper, skin effect becomes significant at much lower frequencies. For example, skin depth in centimeters is given by .delta.=5,033 .rho./.mu.f (Terman, Radio Engineer's Handbook, 1st Edition, p. 34, McGraw-Hill, 1943) where .rho. is resistivity in ohm-cm, .mu. is relative permeability and f is frequency in hertz. For aluminum at 10 Ghz, .delta. is approximately 8.times.10.sup.-5 cm. For silicon having a resistivity of 0.001 ohm-cm, .delta.=1.59.times.10.sup.-3 cm. Thus, if conductor 15 was a layer of aluminum 1.59.times.10.sup.-3 cm wide and 10.sup.-4 cm thick, its series resistance R.sub.S would be about 19.6 ohms per cm. A corresponding layer of silicon, such as substrate 12, would have a series resistance of about 390 ohms per cm, a ratio of nearly 20:1.
On the other hand, if conductor 15 were 10.sup.-4 cm wide and 10.sup.-4 cm thick, the ratio of the series resistance of silicon to the series resistance of the conductor 15 would be reduced, but the high frequency series resistance R.sub.S of the silicon, which is effective in attenuating waves with rise times of 15 picoseconds, would be around 5 to 7 times higher than that of the aluminum. For example, for 50 picosecond rise times, the ratio would probably be in the range 3 to 4.5. Taking 6 as the ratio for a 15 picosecond (10 Ghz) rise time, the substrate series resistance R.sub.S would be around 1800 ohms per centimeter versus 300 ohms per centimeter for an aluminum conductor 10.sup.-4 cm.times.10.sup.-4 cm in area. Using these assumptions, the combined series resistance R.sub.S of the conductor 15 and the substrate would be approximately 2100 ohms/cm.
With the characteristic impedance Z.sub.O of a transmission line composed of the conductor 15 and the substrate 12 in the range of 50-100 ohms, e.g., about 70 ohms, attenuation of a 10 Ghz signal is given approximately by the equation e.sup.-(R s.sup.X/2 Z O) where R.sub.S is the combined series resistance per unit length (i.e., 2100 ohms per cm in this case) and X is the line length. For a 20% attenuation, X=1.49.times.10.sup.-2 cm. For a 50% attenuation, X=4.6.times.10.sup.-2 cm. These lengths are short compared to the lengths of some conductors on complex logic chips. A reduction of R.sub.S is therefore essential for circuits operating at extremely high frequencies.